The present invention relates to an extrusion-type application device. More particularly, the invention relates to an improved application device which has a doctor edge portion at the top of the body of the device and which continuously expels a liquid toward the surface of a moving carrier to apply the liquid at a uniform thickness to the surface of the carrier along the doctor edge portion.
The carriers mentioned herein, and which are sometimes referred to as a flexible sheet or a web, may be materials such as a plastic film, paper, polyolefin-coated paper and a metal sheet of aluminum, copper or the like. The carrier may have an undercoating layer or the like.
The coating liquids mentioned herein may be any of a magnetic liquid, a photographic photosensitive liquid or the like and which is applied to the carrier to produce a magnetic recording medium, a photographic film, photographic printing paper or the like.
Various conventional devices have been used for application to a carrier of the above-described types of liquids. Examples of application devices of the extrusion type and having a doctor edge portion are disclosed in Japanese Unexamined Published Patent Applications Nos. 138036/75 and 84771/80 and in Japanese Patent Application No. 7306/79. Such devices have been used in various fields. However, it is a common disadvantage in the conventional devices that the range in which liquid application can be stably performed is very narrow. Especially, it is very difficult with the conventional devices to stably form a liquid layer of 26 .mu.m or less in thickness on a carrier running at a speed of 100 to 150 m/min or more. As a result of studies carried out by the present applicants, it has been found that this disadvantage occurs due to causes which will now be described.
When the speed of movement of the carrier exceeds about 100 to 150 m/min, the quantity of air taken up between the carrier and the extrusion body of the device increases sharply. It is important to accurately control the pressure of the liquid at the outlet part of the slot of the extrusion body if the liquid is to be uniformly applied to the carrier to form a thin layer thereon when the carrier is moving at a speed of 100 to 150 m/min or more. If the pressure of the liquid is not sufficiently high, air bubbles will be taken up into the layer of the applied liquid on the carrier and/or the applied liquid will flow back upstream to partially separate from the carrier and thus make the thickness of the layer nonuniform. If the pressure of the liquid is too high, the thickness of the layer of the applied liquid, which is then applied in a large quantity, is likely to be made nonuniform along the width of the carrier. It has also been found that these undesirable phenomena much depend on the form of the edge portion of the extrusion body of each of the conventional devices.
In order to prevent such phenomena, an improved application device of the extrusion type has been proposed, as disclosed in the Japanese Unexamined Published Patent Application No. 104666/83. In this device, as shown in FIG. 9 herein, a liquid is continuously expelled from the outlet part of a slot 4 to the surface of a flexible carrier W continuously moving along the surface 6a of a back edge portion 6 and the surface 20 of a doctor edge portion 5 so that the liquid is applied to the surface of the carrier. The cross section of the doctor edge portion 5 is triangularly shaped. As a result, pressure is exerted on the liquid in such a manner that a relatively large liquid part P is formed on the surface 20 of the doctor edge portion 5 at the slot 4 when the liquid is applied to the carrier. Since the relatively large liquid part P is always pressurized during the application of the liquid to carrier, air is likely to be prevented from being taken up and trapped between the carrier and the extrusion body of the device along the surface 6a of the back edge portion 6. For this reason, the liquid can be rapidly applied to the carrier to make a thin uniform layer thereon.
However, streaking occurs in the layer of the applied liquid on the carrier if application is continuously performed by the device over long periods of time. In the case that the product made from the carrier and the liquid applied thereto is a magnetic recording medium, for example, such streaking adversely affects the physical properties of the medium, namely, the S/N ratio, the C/N ratio, and the like. It has been found that when application is continuously performed for a long time, contaminants or foreign matter clinging to the surface of the carrier accumulate in the relatively large liquid part P and cause streaking in the layer of the applied liquid. The contaminants or foreign matter are likely to cling to the surface of the carrier, and cannot be completely removed by rinsing or the like. During application, the contaminants or foreign matter enter the liquid on the extrusion body of the application device. Since the cross section of the doctor edge portion 5 is triangularly shaped so that the portion has a vertex 40, the contaminants or foreign matter are less likely to flow out over the vertex during the application but are accumulated in the relatively large liquid part P to cause streaking in the layer of the applied liquid on the carrier. Particularly when the application is rapidly performed, such streaking is caused even by the momentary catching of a foreign substance in between the carrier and the extrusion body of the application device.
The present inventors have carried out studies in order to provide an application device which has the advantage of the improved application device but which is capable of application without streaking. As a result, they have developed and proposed an application device as disclosed in Japanese Unexamined Published Patent Application No. 238179/85. As shown in FIG. 10, the surface of the doctor edge portion 5 of the device is curved, and a relatively large liquid part P is formed in a pressurized state throughout the application to thus avoid causing streaking and to enable rapid application of the liquid to the surface of a carrier moving at a speed of 300 m/min or less. However, it is technically difficult to make the speed of the rapid application higher than 300 m/min with the conventional device. In other words, air accompanying the carrier is likely to be taken up into the layer of the applied liquid on the carrier in thin-layer rapid application when the speed of the movement of the carrier is 300 m/min or more and the thickness of the applied layer is as ti, small as 10 cc/m.sup.2 or less, although the surface of the doctor edge portion 5 of the body of the device is curved in a prescribed manner to prevent streaking from occurring in the layer, as happens with the application device disclosed in Japanese Unexamined Published Patent Application No. 104666/83. More specifically, in the device disclosed therein it is required that the doctor edge portion 5 be curved such that the radius of curvature of the surface is 2 mm or more. If the surface of the doctor edge portion 5 is curved in such a manner that the radius of curvature is less than 2 mm, the behavior of the carrier is so nonuniform along the width thereof as to make the variations in the thickness of the layer of the applied liquid large. Although the minimum applied quantity of the liquid can be reduced by increasing the tension of the carrier so as to increase the pressure of the liquid, if the radius of curvature is set at such a value as not to cause variations in the thickness of the layer, the tension on the carrier becomes such that the carrier is unavoidably stretched, resulting in a reduced product quality, or possibly even a broken carrier.